To describe the epidemiological and other characteristics of emergency department (ED) presentations diagnosed with acute upper respiratory infection (URI). A retrospective study of patients given an ED diagnosis of acute URI from July 2000 to July 2003 at any of the four metropolitan teaching hospitals in Perth, Western Australia. Acute URI accounted for 3.6% (95% CI, 3.5-3.7) of ED presentations, and 80.7% (95% CI, 80.1-81.3) of these were aged less than 15 years. The most common diagnosis was acute upper respiratory infections of multiple and unspecified sites, followed by croup and acute tonsillitis. Of those with croup, 76.0% (95% CI, 74.7-77.3) presented at night, 67.6% (95% CI, 66.2-69.0) were male and the number of presentations with croup was highest in June 2002. The number of diagnoses of acute tonsillitis did not display a great deal of variation from month to month. Overall, hospital admission was 12.3% (95% CI, 11.8-12.8), with a median length of hospital stay of 1 day (IQR 1.0-2.0). An increase in comorbidity, residing in the most disadvantaged areas, and being a re-presentation increased the odds of being admitted. Further investigation is needed into whether alternative medical care services would be appropriate and acceptable for patients with less severe acute URIs.